1. Field of the Invention
The present invention relates to a semiconductor device provided with a function for controlling by a transistor a current supplied to a load, and in particular relates to a display device having a pixel constituted by a current drive type light emitting element of which luminance changes by current, and a signal line driver circuit thereof. In addition, the invention relates to a driving method of the same. Furthermore, the invention relates to an electronic apparatus having a display portion applying the display device.
2. Description of the Related Art
In recent years, a self-luminous type display device of which pixel is constituted by a light emitting element such as a light emitting diode (LED) has attracted attention. As a light emitting element used in such a self-luminous type display device, an organic light emitting diode (OLED (Organic Light Emitting Diode) which is also called an organic EL element, an Electro Luminescence (EL) element, or the like) has attracted attention and become to be used in an EL display or the like. Since the light emitting element such as an OLED is self-luminous type, the visibility of a pixel is higher as compared to a liquid crystal display, a backlight is not required and the response is fast. In addition, the luminance of the light emitting element is controlled by a current value flowing through it.
As a driving method of expressing the gradation of such a display device, there are a digital method and an analog method. In a digital method, the light emitting element is turned on/off by digital control, and a light emitting time is controlled to express gradation; this method is advantageous in that the uniformity of luminance of each pixel is high, while the frequency is required to be increased and power consumption becomes large. On the other hand, an analog method includes a method of controlling a light emission intensity of the light emitting element by analog control and a method of controlling a light emitting time of the light emitting element by analog control. The method of controlling the light emission intensity by analog control is easily affected by variation of characteristics of a thin film transistor (hereinafter also referred to as a TFT) for each pixel and light emission for each pixel is also varied. Meanwhile, the analog time gradation method controls a light emitting time by analog control, so that the uniformity of luminance of each pixel is high; a display device with the analog time gradation method has been described in Non-Patent Document 1 (see Non-Patent Document 1).
Specifically, in a pixel of the display device described in Non-Patent Document 1, an inverter is constituted by a light emitting element and a transistor for driving the light emitting element. A gate terminal of the driving transistor corresponds to an input terminal of the inverter and an anode of the light emitting element corresponds to an output terminal of the inverter. In the case where an image signal voltage is written to the pixel, the inverter is set to be intermediate between on and off. Then, a triangular wave voltage is input to the pixel in a light emitting period to control an output of the inverter. That is, the output of the inverter which is a potential set to the anode of the light emitting element is controlled, so that whether the light emitting element emits light or not is controlled.    [Non-Patent Document 1] SID 04 DIGEST P1394-1397
The luminosity obtained from one pixel is decreased as the definition of a display device is increased. It is to be noted here that luminosity means the amount showing the degree of luminance of a light source obtained from a certain direction, and besides, luminance means the luminosity of a light source per unit area.
Here, in different pixels, in the case where respective light emitting elements therein have the same luminance, the luminosity obtained from one pixel can be higher as an area of a light emitting region in the pixel is increased. That is, if an aperture ratio which is shown by a ratio of an area of a light transmitting region other than a non-light transmitting region (also called a black matrix) in one pixel to an area of the one pixel is high, desired luminosity can be obtained from the one pixel without increasing a driving voltage even if the area of one pixel is decreased by increasing the definition.
Here, an aperture ratio of a pixel is low if the number of transistors and wires constituting the pixel is large. Thus, it is an object of the invention to decrease the number of transistors and wires constituting a pixel to increase the aperture ratio.